Ejector Pin Marks

Table of Contents

Injection-molded parts are often parts to sell as a brand new product. Customers expect manufactured goods to look perfect. The injection molding process by design should give perfect molded products. But in manufacturing, a few things go wrong leading to imperfections in the product. In a world where customers have thousands of product brands to choose from. Your product has only a few seconds to convince a customer. For products such as phone cases, food containers, aesthetics are important. Slight marks and imperfections in food products give the impression of unhygienic. In products like phones that have become fashion items, look matters. A slight imperfection can be a deciding factor between yours and other products. One of such imperfections is ejector pin marks. Also known as push pins, ejector pin marks can ruin the aesthetic appeal of your plastic part. This article discusses what pin marks are and how to get rid of or prevent them.

How the Ejector Pins work

If we look at the way the ejector pins work, we understand better how ejector pin marks occur. The diagram below gives an illustration of how ejector pins work.  This uses a blade ejector as an example.

Image illustrating ejector pins

When the melt in the mold cools and the product gets formed it is time for product release. Ejector pins play an important role in allowing fast and efficient release. Manual release is often difficult, time-consuming, or even risky. For example in products where sterile conditions need to get maintained. Or sensitive products that release into a close environment. These can be for example to protect from moisture. The ejector pins are projections that exert force on the product. In doing so they push it out of the mold in one axial motion. The motion of the ejector pins gets guided by the clamp plates and ejector box. Once cooling is complete, the retainer plates move out of the way. This allows the mold to push back towards the pins. As the mold pushes down the pins fit into bores in the mold half that fit the pins. The pins then exert a push force on the molded part. This ejects the product. This means that during the ejection, the pins come in contact with the molded part. 

How to Spot Ejector pin marks

It is important to inspect every product for any sign of defects. Where there are large numbers of products made this might not be practical. The approach here is thus to take samples. The sampling strategy depends on the standards and capabilities of the company. For example, it can be one sample in every 100 products made. Or it could be a fixed number of samples taken from a lot at once at daily or weekly intervals. Ejector pin marks manifest in the form of glossy or whitish looking dents on a molded part. These marks occur corresponding to where the ejector pins contacted the product. Knowledge of the location of the pins on the products helps to detect ejector pin marks faster. Other than the dent on the aesthetic appeal, ejector pin marks can also lead to cracks on the product later. So it also affects the long term integrity of the product.

Image showing example of ejector pin marks on the part.

What causes Ejector pin marks

Some problems occur in injection molding and the causes are obvious. While others like ejector pin marks are not so obvious. This is because they can get caused by a combination of factors. They could be a result of inefficiencies in the injection process. Or mold or material selection. So although the ejector pins create the mark, they may be functioning well. The cause of the mark forming most often turns out to lie elsewhere.

Pressure Settings

If the injection pressure is too high this can cause ejector pin marks on the product. The holding pressure is also important. This is the pressure at which the melt gets held while it cools. The higher the pressure, the more difficult the product is to remove. The ejector pin should only have minimal contact with the product. It should be a very brief push, like a tap to get the product out. But if the product is under excessive pressure the contact is more. This contributes to ejector pin marks. Excessive pressure also means that the shrinkage did not have much effect. The little shrinkage that occurs as the product cools helps detach it from the mold cavity walls. This leaves a little gap between the product and the walls. When the melt gets held at excessive pressure the benefit of shrinkage isn’t realized.

Cooling Rate

The cooling rate relates to the initial temperature of the mold in which the melt gets filled. If the mold is too cold this is not good. It results in the outermost part of the product cooling too fast. The entire product shows non-uniform cooling as a result. This causes the product to build up internal stress. Thus when in contact with the mold pin the product is more prone to pin marks. 

Dwell Time

Similarly, if the melt gets left in the mold for too long this gives more chance of ejector pin marks occurring. After the plastic gets mixed and melted in the heated barrel it then gets injected. It gets held here at a constant pressure at a given temperature. The dwell time is usually higher than the glass transition temperature. It varies for different polymers. For example, the dwell temperature for ABS/PVC blend can be around 260oC to 300oC. The dwell time can be between 0 and 5 minutes. This depends on the total cycle time and the goal of the operator. Dwell time has an impact on plastic properties. Of particular importance is the glass transition temperature. The glass transition temperature can get increased by dwell time and/or temperature. A change in the glass transition temperature could mean premature ejection. The product may still not be completely hard upon ejection. Thus leading to ejector pin marks occurring on the part. 


In some mold, designs melt and get into the gaps around the ejector pin holes within the cavity. The excess material formed as a result may stick to the product. Because of the small size of the pins breaking these off is tricky. This is unlike the excess material from the gate or runner. These have inbuilt mechanisms to get broken off. They also get located such that the break-off points are not so noticeable.


You can avoid the ejector pin marks by taking measures to prevent internal stress. Also ensuring that the part cools well and the ejector pins are well designed.  

Ejection System Design

Have a good number of ejector pins, the more the better. It is also important that the pins are well arranged and balanced. When designing ejector pins consider the design of the products. Design the parts to contact the ejector pins with higher thickness. This gives them higher resistance to ejection force. Where you see parts that are prone to resist ejection, arrange more pins in such areas. If the ejector pins are not well aligned this can result in ejector pin marks. Also if the wrong ejector pin speed settings get used. This results in pin mark developing. If the ejection speed is too fast this gives it too high kinetic energy. The balance of forces between the product and the pins can result in reaction force. This reaction force on the pin exerts stress. This can cause the added motion that results in ejector pin marks forming. The speed setting thus needs to be such that the energy of the pin gets balanced by that needed to eject the product. The pins also must get designed to withstand the impact of ejection. 

Choice of Plastic

Where you have various choices of plastics for a product use this chance to avoid ejector pin marks. Go for a plastic that is more abrasion-resistant. Plastics such as nylon, ultrahigh molecular weight polyethylene, and polyoxyethylene. These have good abrasion resistance. So even where the system settings make ejector pin marks likely, the plastic may still resist it. Or it might occur but not be noticeable thus preventing part rejection.

Mold release Agent

Mold release agents are great for ensuring easy and successful ejection of product. They reduce friction between the product and the cavity walls. They also reduce the surface and electrostatic forces. These cause the materials to stick together. But bear in mind that some plastics are not compatible with mold release agents. For example when molding clear plastics. In this case, mold release agents can result in color formation. This is undesirable for clear plastics. So your option here is maintaining the right conditions that ensure good ejection.

Draft Angle

Enabling the easy release of mold reduces the chance of ejector pin marks. Where the product needs excessive force to eject leads to such imperfections. Having enough draft angle on the product makes a lot of difference. Draft angles should get included in the product design to prevent release problems. This means the ejector pins have less work to do and have minimal contact with parts. 

Injection speed

Excessive injection speeds result in high pressure within the mold. As the product cools this results in the build-up of internal stress. The injection speed relates to several other parameters. These include injection pressure, gate design, runner dimensions, and melt viscosity. By controlling these parameters you can reduce the occurrence of ejector pin marks. The general idea is to ensure optimal mold design that allows for a good filling. The melt should fill in such a way to avoid internal stress and other imperfections. For example, a wider gate allows more fluid to flow into the mold at a given time. 

Cooling Time

If the mold surface is not completely cooled, it is more prone to abrasion. Plastics achieve their full characteristics after they solidify. If the cooling in the mold is inefficient then the ejection is premature. Ensuring that proper cooling occurs and the product has gotten enough time to shrink is important. The shrinkage also aids in the product release. Operators. must know the difference between points E and F. The point where the product hardens and when it cools. The image below explains this.

Image showing the heating (red line) and cooling (Blue line) of the product. Between point A to B the product absorbs heat and temperature increases. This occurs up to point A. When it reaches its softening point it begins to melt. A to B indicates glass transition temperature. In injection molding, this occurs in the heated barrel. When the melt gets injected, the heating stops. This is at point C. By the time it passed through the runner and gate temperature had begun to decrease. The melt temperature begins to drop until point D. by point E the product has hardened. But it is not until point F that shrinkage occurs.  

Reduce Dwell time

Other than shrinkage the dwell time should also get considered during cooling. In setting the cooling time you should consider the dwell time set. The cooling time must correspond to the change in the properties caused by dwell time. This also needs a good understanding of the plastics. Dwell time does not affect some plastics but does have a significant effect on others. Excessive dwell time can cause changes to the properties of the plastics. These can alter the behavior of the plastic and make ejector pin marks more likely.

Mold design

The mold design affects the efficiency of product release. Ensure that the surface of the mold is well polished. The mold should also get designed with the product release in mind. The passing of the cooling channels should ensure optimal cooling. Where possible use conformal cooling channels. The positioning of the ejector pins should be in consideration. The mold should also avoid any leakage through the pinholes. In other words, ensuring that the mold is fit to purpose.


The ejection of the product is the final stage of injection molding. Here any inefficiencies in the earlier stages of injection molding imperfection. Ejector pin marks occur as a result of a combination of factors. Either or all the factors discussed within this article can lead to ejector pin marks. Proper design of ejector pins, material selection, adequate process settings amongst others. These can prevent ejector pin marks.

At we design optimized molds. We give careful considerations to prevent the occurrence of imperfections like pin marks. Discuss your injection molding project with us. Take a look at the range of injection molding services and solutions we offer.


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